Then the correct approach is "you are likely not ready for this material, I recommend you do XYZ and come back". There's no problem with telling a reader they

Then the correct approach is "you are likely not ready for this material, I recommend you do XYZ and come back". There's no problem with telling a reader they are not ready for a text - in fact, it can save them time. The way in which you do it though can encourage or discourage a student.